Polystyrene foam exhibits excellent cushioning, heat insulating properties, and easy molding and is widely employed as packaging material or heat insulating material. However, it is liable to partially break off due to insufficient impact resistance or softness so that it is unsuitable for, for example, packaging of precision components. Although expanded polyolefin resins have excellent impact resistance and softness, they require large-scale productive facilities, and they must be, by nature, transported from a manufacturer to a molder in the form of expanded beads, which incurs increase of production cost.
In recent years, expanded materials obtained from a rubber-modified styrene resin which is prepared by bulk polymerization or suspension polymerization of a styrene monomer in the presence of butadiene rubber as a base resin have been proposed for easy molding and improved impact resistance and softness over the conventional polystyrene resin foam, as disclosed in JP-B-47-18428 (the term "JP-B" as used herein means an "examined published Japanese patent application") and JP-B-51-46536. However, the degree of the improvements in these materials is still insufficient.